1. Field of the Invention
The present invention relates generally to the field of digital communications. Specifically, the present invention relates to error concealment in digital communications.
2. Description of the Related Art
Systems for wireless transmission of audio signals are known in the art. FIG. 1 shows a block diagram of a typical structure for such a system 100. The audio equipment 102 is an audio source and may be, for example, a microphone, an FM tuner/receiver, or an analog recording media. The audio signal from the audio equipment 102 is provided to an audio analog to digital converter (ADC) 104 for conversion to digital samples. The digital samples are then provided to digital modem transmitter (TX) 106. Digital modem transmitter 106 may include a formatting unit (not shown) for formatting the digital samples into a specific format. The digital modem transmitter 106 may also include a forward error correction (FEC) coder (not shown) for adding error correction codes to the formatted data and a modulator (not shown) for modulating the formatted data onto a carrier signal. The output of the modulator may be provided to a filter (not shown) and converted to an analog signal using a digital to analog converter (DAC) (not shown) before being sent to the radio frequency (RF) transmitter 108 for transmission via antenna 110.
On the receiver side, the transmitted signal will be received at the RF receiver (RX) 114 via antenna 112. The received signal is then provided to digital modem receiver 116. Digital modem receiver 116 may include ADCs (not shown) for digitizing the received signal and a demodulator (not shown) for recovering the audio data in digital format. Digital modem receiver 116 may further include an FEC decoder (not shown) for recovering the originally transmitted data and a de-formatter (not shown) for de-formatting the demodulated data. Digital modem receiver 116 then provides the demodulated and de-formatted data to audio DAC 118 for conversion to an analog signal that is reproduced by the speaker 120.
A problem involved in the transmission of audio data is that the modulated carrier signal transmitted by the RF transmitter 108 shown in FIG. 1 is subject to corruption of the digitized data during transmission due, for example, to close proximity of other transmission sources. For example, noise may corrupt the signal transmitted by RF transmitter 108 and may result in un-recovered data at the RF receiver 114 due to errors in the data. The result of such a situation may be annoying cracks, pops and the like in the speaker 120.
One method for minimizing such negative effects of corruption of the modulated carrier signal is to provide a level of redundancy for the digitized samples. Such redundancy may be provided by data coding. Data coding consists of adding redundant information to source data in such a way that errors in a received encoded data stream may be identified and corrected. The degree of redundancy that is added determines the number of errors in the received signal that may be corrected, before an uncorrectable level of data interference occurs. Typical coding techniques identify and correct individual bit errors in data streams. As an example of data coding, the FEC coder described above may implement forward error correction codes using well known codes such as convolutional, Bose-Chaudhuri-Hocquenghem (BCH), Reed-Solomon Coding, and the like, in order to eliminate errors of relatively short duration.
Another technique, known as interleaving, may be used. Interleaving processes may be used to separate the digitized samples of the transmitted sequence that would normally be transmitted together. Interleaving processes known in the art reorder the data before transmission such that, for example, any two successive data symbols are separated by a particular number of data symbols in the transmitted sequence. Upon reception, the data symbols are de-interleaved to their original order before the data is recovered. Thus, the interleaving process allows a greater probability that a larger number of errors in the transmitted sequence may be corrected by spreading the errors across the transmitted sequence.
As a result of interleaving a data signal, digitized sample corruption imposed upon this type of interleaved data signal does not affect contiguous bits from the original transmitted sequence, and after de-interleaving, the received coded data will have bit errors that are spread out over time and which may be correctable using the redundant data that was added at the data coding stage. Thus, at the receiver, the group of interleaved digitized samples with the error correction codes are checked, and the group of interleaved digitized samples with errors are corrected.
However, even such sophisticated error codes cannot correct all the bit errors in the digitized samples. In the case where digitized samples are uncorrectable, another technique, known as interpolation, may be used in an attempt to conceal uncorrectable digitized samples. Interpolation processes known in the art estimate a sample value of those uncorrectable digitized samples by interpolating from adjacent digitized samples to conceal any effect of the uncorrectable digitized samples.
However, interpolation may not work well where there are many errors in a transmitted sequence, as there may not be enough good samples from which to interpolate. In other words, interpolation techniques require enough good samples to estimate or predict a sample that is missing as a result of an error. As an example, a first-order interpolator will not be effective when there are sequentially adjacent samples having errors (i.e., two sequentially adjacent bad samples), as linear interpolation works by interpolating the bad sample from two adjacent good samples. Higher-order interpolators may be used to improve the effectiveness of the interpolation process where there are errors in sequentially adjacent samples. However, even higher-order interpolators are limited in their ability to conceal errors under conditions where there are multiple errors.
Any errors in the digitized samples that cannot be corrected or concealed by the techniques discussed above may be muted. However, sudden activation of muting may be distracting and annoying to a user listening to the reproduced audio signal.
Thus, there is a need for a low complexity apparatus, system and method for the concealment of errors such that high perceptual audio quality is achieved.